This invention relates to materials for medical or veterinary use that possess considerably improved properties with regard to friction. The invention also relates to the production of such materials from existing materials that possess suitable bulk properties, but inappropriate frictional properties. This invention also relates to a hydrophilic coating material that provides improved frictional properties.
In a particular aspect, the materials and method of this invention are useful for the fabrication of articles which contact the fluids or tissues of humans or animals.
In a medical procedure the facile movement of a surface with respect to tissue is important in reducing damage to both the material of the Surface and to the tissue. Damage to tissue as a result of "tissue drag" causes inflammation and pain to the person or animal and leads to a longer recovery time. High friction between the surface material and blood may result in clotting and subsequent occlusion of a blood vessel, for example with indwelling catheters. Friction may also damage the material, thus rendering it ineffective or shortening its useful life. The problem of "tissue drag" has been of concern to the medical and veterinary professions for some time.
Although friction may be reduced by the use of lubricating oils, such liquids are weakly associated with the surface of the material and will be removed upon repeated rubbing of the surface against the tissue, as in multiple loop suturing. The toxicity of the oils and their breakdown products are a major concern. The surfaces Of this invention are an improvement over the existing art which uses these lubricating oils.
A number of patents describe the use of non-absorbable hydrophilic or hydrogel coatings which provide lubrication when wet. The coatings are disadvantageous due to the complex coating methods described. The multiple components of these coatings would also present serious toxicology concerns.
The frictional properties of solid polymers, metals or ceramics are strongly dependent on the chemical composition of the material surface. The tissue drag of medical or veterinary materials is a result of interfacial forces between the material and the tissue, and as a result is also affected by the composition of the material surface. A number of techniques known in the art are designed to effect a chemical change in the surface of materials and thereby alter the frictional properties of the surface. These techniques include corona discharge, flame treatment, acid etching, etc.
The present invention will be illustrated with respect to elastomeric sutures. It should be understood, however, that this in no way limits the scope of the invention. The method of the invention is equally useful for imparting low friction surfaces to catheters, pacemaker leads and other medical or veterinary products.
The utility of this invention is illustrated by the following analysis of suture properties as related to continuous vascular anastomosis: "The use of monofilament polypropylene (PROLENE.RTM.-Ethicon) or monofilament polybutester (NOVAFIL.RTM.-Davis and Geck) makes accurate anastomosis feasible, by allowing a number of suture loops to be placed with good vision prior to tightening of the suture to approximate the edges to be joined. Monofilament polypropylene (PROLENE.RTM.) has long given excellent results in our experience. The particular advantage of this material is the ease with which the suture will pull through the tissues with minimal tissue drag. PROLENE.RTM. is relatively stiff and even in fine anastomoses with 7/0 PROLENE.RTM. there may be distortion of the anastomotic line. The recently available monofilament polybutester (NOVAFIL.RTM.) has given excellent results in our practice and has shown improved handling characteristics--less `memory` and stiffness making it an easier material to use. The suture is slightly elastic--this may have theoretical advantages in a pliant small vessel--and the reduced stiffness results in less distortion of fine anastomotic lines. When using the open technique, it is important to pull the suture tight after every two or three loops to avoid having loose loops in the anastomosis. This overcomes any problem that may be experienced with increased tissue resistance when using NOVAFIL.RTM.." (From "Surgery of Coronary Artery Disease", D. J. Wheatley, ed., The C. V. Mosby Co., St. Louis, 1986.)
In certain procedures the surgeon, rather than tightening each suture loop individually may place multiple loose loops of suture in the tissue and then tighten the loops by pulling one or both ends (see FIG. 1). In vascular and other tissues, the two ends of tissue may be pulled together by pulling the suture ends, which is often referred to as the "parachute technique." The parachute technique is commonly used in suturing vascular tissue. Polypropylene is the most common suture material used for this technique. Techniques of this sort are advantageous since they provide benefits to the patient. Suture placement is more accurate since each end of tissue is better visualized. The technique is also faster and reduces the time until blood flow is restored.
Good slip of sutures through tissues is critical for applications such as vascular surgery. However, some currently available sutures such as polybutester tend to exhibit "stick-slip" behavior as they are pulled through tissue. These surfaces may initially move smoothly, but they then stick to the tissue and have to be pulled harder in order to continue the suturing process. The resulting oscillatory force may damage delicate vascular tissue. Sutures made from materials such as polypropylene do not show stick-slip behavior to the same extent as polybutester, but they are not sufficiently elastic for surgery on delicate tissue. A particular aspect of this invention overcomes the deficiencies of the prior art by disclosing a method for fabricating sutures which are highly elastic and have a low friction surface that avoids stick-slip behavior.
There is an advantage in using elastomeric sutures such as polybutester in cardiovascular surgery, in that an anastomosis (the sutured junction between two vessels) reflects more closely the natural compliance of the anatomical vessel. Such simulation of the properties of the natural vessel should help maintain long term patency of the anastomosis after surgery, in contrast to presently used non-elastic sutures which have high long term occlusion rates. A further advantage includes the superior ability of polybutester to maintain strength after gamma-ray sterilization as opposed to many other commonly used elastomeric sutures.
The stick-slip phenomenon is especially acute when either the suture or the tissue is elastic and when the suture is moving slowly. The problem is accentuated when there are multiple loops of the suture through the tissue (FIG. 1). To overcome this deficiency, the present invention provides a coating to the surface of known polymers. Similarly, these coatings can provide improved performance to metal or ceramic surfaces.
Accordingly, in one aspect, the present invention provides a novel, multilayer composite material with low tissue drag, the composite comprising a thin, polymeric surface layer and a bulk material of the required properties and form. The thin polymeric layer consists of a hydrophilic block copolymer suitable for contact with human or animal tissue.
In a second aspect, the invention provides a suture comprised of a multilayer composite material according to the first aspect of the present invention where the bulk material is an absorbable or nonabsorbable material suitable for suture applications. In a preferred case the suture is fabricated from an elastomeric material.
In a third aspect, the present invention provides a catheter comprising a polymeric article having a thin, polymeric surface layer and a bulk polymeric material, said surface layer providing hydrophilic material for contact with tissues or body fluids.
In yet a further aspect, the present invention provides a medical or veterinary implant with a low friction surface, said implant comprising a material wherein a surface of said materials bears a layer formed by coating the surface with a hydrophilic block copolymer. Such medical or veterinary implants could include absorbable or non-absorbable meshes, pins and screws for orthopedic surgery, vascular grafts, films for prevention of post-surgical adhesions, implants for joint lubrication. In some cases the entire implant could consist of the absorbable hydrophilic copolymers of the invention.
A number of hydrophobic absorbable coatings are disclosed in printed publications. These coatings were applied primarily for improvement of knot tying characteristics of multifilamentary sutures. The following U.S. Pat. Nos. are illustrative: 4,791,929, 4,788,979, 4,705,820, 4,624,256, 4,201,216, and 4,994,074; and EP applications 436308, EP 411545, EP 376656.
One patent for improvement of knot tying characteristics contained a hydrophilic polymer component blended with two hydrophobic components (U.S. Pat. No. 4,027,676).
Several patents exemplify sutures with absorbable hydrophilic coatings for the primary purpose of improving knot tying characteristics: U.S. Pat. Nos. 4,857,602, 4,649,920, 4,047,533, 4,043,344. Two patents exemplify hydrophilic non-absorbable coatings to improve wettability and smoothness, GB 1248513, or to provide for controlled release of an anti-microbial agent, U.S. Pat. No. 4,875,479.
A number of patents disclose the use of non-absorbable hydrophilic coatings for medical articles for the purpose of lubrication: U.S. Pat. Nos. 5,041,100, 4,976,703, 4,961,954, 4,835,003, 4,801,475, 4,743,673, 4,729,914, 4,666,437, 4,589,873, 4,585,666, 4,487,808, 4,373,009, 4,100,309, 4,459,317, 4,487,808, and 4,729,914. Two additional published patent applications disclose hydrophilic lubricant coatings based on water soluble film forming polymers: EP 14238 and WO 8810284 A1.
Several patents disclose the preparation of synthetic absorbable hydrogel polymers. One of these patents, U.S. Pat. No. 4,716,203, discloses the use of a synthetic absorbable hydrogel polymer for the improvement of knot tying characteristics. The remaining patents describe the preparation of synthetic absorbable hydrogels: U.S. Pat. Nos. 4,942,035, 4,826,945, 4,526,938, 4,452,973, 4,438,253. These patents do not disclose the use of the exemplified materials as suture coatings for tissue drag reduction. It is not necessarily true that a hydrophilic or hydrogel polymers will perform well as a coating for tissue drag reduction, since the coating must possess the characteristics of low friction with tissue combined with good adherence as is demonstrated when friction does not increase as the material is repeatedly rubbed against tissue.